1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, etc. for forming an image by electrophotography. Particularly, it relates to an image forming apparatus using toner for forming an image.
2. Description of the Related Art
In electrophotography, a charging unit such as a charging roller is used for giving electric charge on a photosensitive member evenly. An exposure unit is used for forming an electrostatic latent image corresponding to image data on the photosensitive member to which electric charge is given. Toner which is powder electrostatically charged in accordance with the charge pattern of the electrostatic latent image is deposited on the photosensitive member to thereby form a toner image on the electrostatic latent image. The toner image is transferred onto a recording medium such as a sheet of paper directly or through an intermediate transfer belt as an intermediate transfer member to thereby form an image.
Because electric charge remains on the photosensitive member after toner is transferred onto the recording medium, the residual electric charge is erased by an erasing unit before an image is formed again. A method of erasing electric charge by light irradiation or a method of erasing electric charge by an electrically conducting brush etc. brought into contact with the photosensitive member has been proposed as the erasing unit.
For formation of a color image, color toners such as yellow (Y), magenta (M), cyan (C), black (K), etc. are superposed on one another to form an image.
The color image forming method is roughly classified into two techniques. One is a repetitive development technique for forming a color image by repetitively developing the respective color toners on one photosensitive member. The other is a simultaneous development technique for forming a color image by simultaneously developing the respective color toners on a plurality of photosensitive members.
The repetitive development technique uses one photosensitive member for forming a color image. An intermediate transfer technique is an example of the repetitive development technique. The intermediate transfer technique is a technique in which developers that develops different color toners and a medium conveyance member are disposed around a photosensitive member so that toner images formed on the photosensitive member are color by color transferred onto the medium conveyance member successively, for example, as described in JP-A-8-137179. After this operation is repeated color by color so that the toner images of the different colors are superposed on the medium conveyance member, the superposed color toner image formed on the medium conveyance member is transferred onto a sheet of paper to thereby output a color image.
In this technique, for example, toner images of the four colors of yellow, magenta, cyan and black are color by color formed on the photosensitive member successively and then superpositively transferred onto the medium conveyance member. After all the toner images are transferred onto the medium conveyance member, the superposed color image formed on the medium conveyance member is transferred onto a medium such as a sheet of paper. Because colors are superposed successively in this manner, a time about four times as much as the printing time in the case of formation of a monochrome image from a color of black is required for forming an image.
It is however possible to reduce the number of parts because a charging unit, an exposure unit, a transfer unit, a cleaner unit and an erasing unit necessary for printing and developers corresponding to the four colors can be formed around one photosensitive member.
On the other hand, in the simultaneous development technique, photosensitive members corresponding to colors are provided as described in JP-A-5-35097. Toner images are almost simultaneously formed on the photosensitive members. The toner images are transferred in accordance with conveyance of a sheet of paper to thereby form a color image. This technique is also called “tandem electrophotographic”.
In the tandem electrophotographic, image forming section each having a photosensitive member, a charging unit, an exposure unit, a developing unit and a cleaner unit are provided independently in accordance with the colors. Accordingly, when a color image is formed from toners of the four colors of yellow, magenta, cyan and black, it is necessary to provide four image forming sections.
After toner images are almost simultaneously formed by the independent image forming section corresponding to the four colors, the toner images are transferred onto an intermediate transfer medium or a medium such as a sheet of paper. Because colors are superposed simultaneously in this manner, a color image can be formed in a time approximately as much as the printing time in the case of formation of a monochrome image from a color of black. This technique is suitable for high-speed printing of a color image.
It is however necessary to increase the number of parts because all printing processes required for forming images corresponding to the colors must be prepared.
A method of detecting the amount of toner remaining in a developer has been described in JP-A-5-35097. A residual toner amount detection unit includes a toner sensor, and toner detection windows. The toner sensor is disposed in an image forming apparatus and has a light-emitting element, and a light-receiving element. The toner detection windows are provided in the developer.
When there is no toner in the developer, light emitted from the light-emitting element is transmitted through one of the toner detection windows in the developer and further transmitted through the other toner detection window on the opposite side via the inside of the developer so that the light is received by the light-receiving element.
When there is toner in the developer, light emitted from the light-emitting element is blocked by the toner in the developer so that the light cannot reach the light-receiving element on the opposite side. On this occasion, the presence of toner can be detected because the light-receiving element outputs a voltage or the like proportional to the quantity of light received by the light-receiving element.
A method of erasing residual electric charge has been described in JP-A-11-344909. In this method, a light-emitting element such as an LED lamp or a fluorescent lamp is disposed between a transfer unit and a charging unit so that electric charge remaining on a photosensitive member can be erased by application of light on the photosensitive member.
In recent years, color printers have become widespread rapidly and reduction in cost of the printers has advanced because of increasing demands for colorization of documents in offices. In addition, tandem color printers have attracted notice because of demands for increase in printing speed.
It is however difficult to reduce the size of the tandem printer because the tandem printer must have four image forming sections as descried above. The tandem printer requires a large number of parts. Accordingly, the size of the tandem printer and the cost of parts in the tandem printer must become larger than those of a printer using the repetitive development technique. Above all, the residual toner amount detection unit needs light-emitting element and light-receiving element pairs, light-emitting element drive circuits and light-receiving element receiving circuits in accordance with the four colors, so that increase in production cost is brought.